When two or more consonants come together they are called conjuncts. They sometimes change their shape and sometimes retain their shape.
Although all consonant combinations are possible, not all are used in everyday use. This page gives all the conjuncts.
For some we can guess from the shape whereas for some which change shape it is hard to decipher. It is necessary to learn the hard to decipher conjuncts.
We will be dealing with hard to decipher conjuncts later.